Symmeric sweep phase sweep transmit Diversity

ABSTRACT

Described herein is a method and apparatus for transmission that provides the performance of space time spreading (STS) or orthogonal transmit diversity (OTD) and the backwards compatibility of phase sweep transmit diversity (PSTD) without degrading performance of either STS or PSTD using a symmetric sweep PSTD transmission architecture. In one embodiment, a pair of signals s 1 , and s 2  are split into signals s 1 (a) and s 1 (b) and signals s 2 (a) and s 2 (b), respectively. Signal s 1  comprises a first STS/OTD signal belonging to an STS/OTD pair, and signal s 2  comprises a second STS/OTD signal belonging to the STS/OTD pair. Signals s 1 (b) and s 2 (b) are phase swept using a pair of phase sweep frequency signals that would cancel out any self induced interference. For example, the pair of phase sweep frequency signals utilize a same phase sweep frequency with one of the phase sweep frequency signals rotating in the opposite direction plus an offset of π relative to the other phase sweep frequency signal. The resultant phase swept signals s 1 (b) and s 2 (b) are added to signals s 2 (a) and s 1 (a) before being amplified and transmitted.

RELATED APPLICATION

[0001] Related subject matter is disclosed in the following applicationsfiled concurrently and assigned to the same assignee hereof: U.S. patentapplication Ser. No. ______ entitled, “Space Time Spreading and PhaseSweep Transmit Diversity,” inventors Roger Benning, R. Michael Buehrer,Paul A Polakos and Robert Atmaram Soni; U.S. patent application Ser. No.______ entitled, “Biased Phase Sweep Transmit Diversity,” inventorsRoger Benning, R Michael Buehrer and Robert Atmaram Soni; and U.S.patent application Ser. No. ______ entitled, “Split Shift Phase SweepTransmit Diversity,” inventors Roger Benning, R. Michael Buehrer, RobertAtmaram Soni and Paul A Polakos.

Background of the Related Art

[0002] Performance of wireless communication systems is directly relatedto signal strength statistics of received signals. Third generationwireless communication systems utilize transmit diversity techniques fordownlink transmissions (i.e., communication link from a base station toa mobile-station) in order to improve received signal strengthstatistics and, thus, performance. Two such transmit diversitytechniques are space time spreading (STS) and phase sweep transmitdiversity (PSTD).

[0003]FIG. 1 depicts a wireless communication system 10 employing STS.Wireless communication system 10 comprises at least one base station 12having two antenna elements 14-1 and 14-2, wherein antenna elements 14-1and 14-2 are spaced far apart for achieving transmit diversity. Basestation 12 receives a signal S for transmitting to mobile-station 16.Signal S is alternately divided into signals S_(e) and s_(o) whereinsignal s_(e) comprises even data bits and signal s_(e) comprises odddata bits. Signals s_(e) and s_(o) are processed to produce signalsS¹⁴⁻¹ and S¹⁴⁻². Specifically, s_(e) is multiplied with Walsh code w₁ toproduce signal s_(e)w₁; a conjugate of signal s_(o) is multiplied withWalsh code w₂ to produce signal s_(o)*w₂; signal s_(o) is multipliedwith Walsh code w₁ to produce s_(o)w₁; and a conjugate of signal s_(e)is multiplied with Walsh code w₂ to produce s_(e)*w₂. Signal s_(e)w₁ isadded to signal s_(o)*w₂ to produce signal S¹⁴⁻¹ (i.e.,S¹⁴⁻¹=s_(e)w₁+s_(o)*w₂) and signal s_(e)*w₂ is subtracted from signals_(o)w₁ to produce signal S¹⁴⁻² (i.e., S¹⁴⁻²=s_(o)w₁−s_(e)*w₂). SignalsS¹⁴⁻¹ and S¹⁴⁻² are transmitted at substantially equal or identicalpower levels over antenna elements 14-1 and 14-2, respectively. Forpurposes of this application, power levels are “substantially equal” or“identical” when the power levels are within 1% of each other.

[0004] Mobile-station 16 receives signal R comprisingγ₁(S¹⁴⁻²)+γ₂(S¹⁴⁻²), wherein γ₁ and γ₂ are distortion factorcoefficients associated with the transmission of signals S¹⁴⁻¹ and S¹⁴⁻²from antenna elements 14-1 and 14-2 to mobile-station 16, respectively.Distortion factor coefficients γ₁ and γ₂ can be estimated using pilotsignals, as is well-known in the art. Mobile-station 16 decodes signal Rwith Walsh codes w₁ and w₂ to respectively produce outputs:

W ₁=γ₁ s _(e)+γ₂ s _(o)  equation 1

W ₂=γ₁ s _(o)*−γ₂ s _(e)*  equation 1a

[0005] Using the following equations, estimates of signals s_(e) ands_(o), i.e., ŝ_(e) and ŝ_(o), may be obtained:

ŝ _(e)=γ₁ *W ₁−γ₂ W ₂ *=s _(e)(|γ₁|²+|γ₂|²)+noise  equation 2

ŝ _(o)=γ₂ *W ₁+γ₁ W ₂ *=s _(o)(|γ₁|²+|γ₂|²)+noise′  equation 2

[0006] However, STS is a transmit diversity technique that is notbackward compatible from the perspective of the mobile-station. That is,mobile-station 16 is required to have the necessary hardware and/orsoftware to decode signal R. Mobile-stations without such hardwareand/or software, such as pre-third generation mobile-stations, would beincapable of decoding signal R.

[0007] By contrast, phase sweep transmit diversity (PSTD) is backwardcompatible from the perspective of the mobile-station. FIG. 2 depicts awireless communication system 20 employing PSTD. Wireless communicationsystem 20 comprises at least one base station 22 having two antennaelements 24-1 and 24-2, wherein antenna elements 24-1 and 24-2 arespaced far apart for achieving transmit diversity. Base station 22receives a signal S for transmitting to mobile-station 26. Signal S isevenly power split into signals s₁ and s₂ and processed to producesignals S²⁴⁻¹ and S²⁴⁻², where s₁=s₂- Specifically, signal s₁ ismultiplied by Walsh code w_(k) to produce S²⁴⁻¹=s₁w_(k), where krepresents a particular user or mobile-station. Signal s₂ is multipliedby Walsh code w_(k) and a phase sweep frequency signal e^(j2πf) ^(_(s))^(t) to produce S²⁴⁻², i.e., S₂₄₋₂=s₂w_(k)e^(j2πf) ^(_(s))^(t)=s₁w_(k)e^(j2πf) ^(_(s)) ^(t)=S²⁴⁻¹e^(j2πf) ^(_(s)) ^(t), where _(s)is a phase sweep frequency and t is time.

[0008] Signals S²⁴⁻¹ and S₂₄₋₂ are transmitted at substantially equalpower levels over antenna elements 24-1 and 24-2, respectively. Notethat the phase sweep signal e^(j2πf) ^(_(s)) ^(t) is being representedin complex baseband notation, i.e., e^(j2πf) ^(_(s))^(t)=cos(2πf_(s)t)+jsin(2πf_(s)t). It should be understood that thephase sweep signal may also be applied at an intermediate frequency or aradio frequency.

[0009] Mobile-station 26 receives signal R comprising γ₁S²⁴⁻¹+γ₂S²⁴⁻².Simplifying the equation for R results in

R=γ ₁ S ²⁴⁻¹+γ₂ S ²⁴⁻¹ e ^(j2πf) ^(_(s)) ^(t)  equation 3

R=S ²⁴⁻¹{γ₁+γ₂ e ^(j2πf) ^(_(s)) ^(t)}  equation 3a

R=S ²⁴⁻¹γ_(eq)  equation 3b

[0010] where γ_(eq) is an equivalent channel seen by mobile-station 26.Distortion factor coefficient γ_(eq) can be estimated using pilotsignals and used, along with equation 3b, to obtain estimates of signals₁ and/or S₂.

[0011] In slow fading channel conditions, both transmit diversitytechniques, i.e., STS and PSTD, improve performance (relative to when notransmit diversity technique is used) by making the received signalstrength statistics associated wit a slow fading channel at the receiverlook like those associated with a fast fading channel. However, PSTDdoes not provide the same amount of overall performance improvement asSTS. Accordingly, there exists a need for a transmission technique thatprovides the performance of STS and the backwards compatibility of PSTDwithout degrading performance of either STS or PSTD.

SUMMARY OF THE INVENTION

[0012] The present invention is a method and apparatus for transmissionthat provides the performance of space time spreading (STS) ororthogonal transmit diversity (OTD) and the backwards compatibility ofphase sweep transmit diversity (PSTD) without degrading performance ofeither STS or PSTD using a symmetric sweep PSTD transmissionarchitecture, which involves phase sweeping a pair of signals having apair of STS/OTD signals. In one embodiment, a pair of signals s₁ and s₂are split into signals s₁(a) and s₁(b) and signals s₂(a) and s₂(b),respectively. Signal s₁ comprises a first STS/OTD signal belonging to anSTS/OTD pair, and signal s₂ comprises a second STS/OTD signal belongingto the STS/OTD pair. Signals s₁(b) and s₂(b) are phase swept using apair of phase sweep frequency signals that would cancel out any selfinduced interference caused by phase sweeping both signals s₁(b) ands₂(b). For example, the pair of phase sweep frequency signals utilize asame phase sweep frequency with one of the phase sweep frequency signalsrotating in the opposite direction plus an offset of π relative to theother phase sweep frequency signal. The resultant phase swept signalss₁(b) and s₂(b) are added to signals s₂(a) and s₁(a) before beingamplified and transmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The features, aspects, and advantages of the present inventionwill become better understood with regard to the following description,appended claims, and accompanying drawings where

[0014]FIG. 1 depicts a wireless communication system employing spacetime spreading techniques in accordance with the prior art;

[0015]FIG. 2 depicts a wireless communication system employing phasesweep transmit diversity in accordance with the prior art; and

[0016]FIG. 3 depicts a base station employing symmetric sweep phasesweep transmit diversity in accordance with one embodiment of thepresent invention;

[0017]FIG. 4 depicts a base station employing symmetric sweep phasesweep transmit diversity in accordance with another embodiment of thepresent invention; and

[0018]FIG. 5 depicts a base station employing symmetric sweep phasesweep transmit diversity in accordance with another embodiment of thepresent invention.

DETAILED DESCRIPTION

[0019]FIG. 3 depicts a base station 30 employing symmetric sweep phasesweep transmit diversity in accordance with the present invention,wherein symmetric sweep phase sweep transmit diversity utilizes codedivision multiple access (CDMA), phase sweep transmit diversity (PSTD),and space time spreading (STS) or orthogonal transmit diversity (OTD)techniques. CDMA, PSTD, STS and OTD are well-known in the art.

[0020] Base station 30 provides wireless communication services tomobile-stations, not shown, in its associated geographical coverage areaor cell, wherein the cell is divided into three sectors α, β, γ. Notethat the base station could be divided into an arbitrary number ofsectors and not change the invention described here. Base station 30includes a transmission architecture that incorporates STS or OTD andbiased PSTD, as will be described herein.

[0021] Base station 30 comprises a processor 32, splitters 34, 35,multipliers 36, 38, 40, 41, adders 42, 43, amplifiers 44, 46, and a pairof diversity antennas 48, 50. Note that base station 30 also includesconfigurations of splitters, multipliers, adders, amplifiers andantennas for sectors β, γ that are identical to those for sector α. Forsimplicity sake, the configuration for sectors β, γ are not shown.Additionally, for discussion purposes, it is assumed that signals S_(k)are intended for mobile-stations k located in sector α and, thus, thepresent invention will be described with reference to signals S_(k)being processed for transmission over sector α.

[0022] Processor 32 includes software for processing signals S_(k) inaccordance with well-known CDMA and STS/OTD techniques. The manner inwhich a particular signal S_(k) is processed by processor 32 depends onwhether mobile-station k is STS/OTD compatible, i.e., mobile-stationcapable of decoding signals processed using STS/OTD. Processor 32 mayalso include software for determining whether mobile-station k isSTS/OTD compatible. If mobile-station k is not STS/OTD compatible, thensignal S_(k) is processed in accordance with CDMA techniques to producesignal S_(k-1), which is also referred to herein as a non-STS/OTD signalS_(k-1).

[0023] Note that, in another embodiment, processor 32 is operable toprocess signals S_(k) in accordance with a multiple access techniqueother than CDMA, such as time or frequency division multiple access. Inthis embodiment, when mobile-station k is not STS/OTD compatible, thensignal S_(k) is processed in accordance with such other multiple accesstechnique to produce the non-STS/OTD signal S_(k-1).

[0024] If mobile-station k is STS/OTD compatible, then signal S_(k) isprocessed in accordance with CDMA and STS/OTD. Specifically, ifmobile-station k is STS compatible, then signal S_(k) is processed usingSTS. Such process includes alternately dividing signal S_(k) intosignals s_(e) and s_(o), wherein signal s_(e) comprises even data bitsand signal s_(o) comprises odd data bits. Signal s_(e) is multipliedwith Walsh code w₁ to produce signal s_(e)w₁, and a conjugate of signals_(e) is multiplied with Walsh code w₂ to produce s_(e)*w₂. Signal s_(o)is multiplied with Walsh code w₁ to produce s_(o)w₁, and a conjugate ofsignal s_(o) is multiplied with Walsh code w₂ to produce signals_(o)*w₂. Signal s_(e)w₁ is added to signal s_(o)*w₂ to produce signalS_(k-2)(a)=s_(e)w₁+s_(o)*w₂. Signal s_(e)*w₂ is subtracted from signals₀w₁ to produce signal S_(k-2)(b)=s_(o)w₁−s_(e)*w₂ Signals S_(k-2)(a),S_(k-2)(b) are also referred to herein as STS signals, and togethersignals S_(k-2)(a), S_(k-2)(b) collectively comprise an STS pair.

[0025] If mobile-station k is OTD compatible, then signal S_(k) isprocessed using OTD. Orthogonal transmit diversity involves dividingsignal S_(k) into signals s_(e) and s_(o), and multiplying signals s_(e)and s_(o) using Walsh codes w₁, w₂ to produce signals S_(k-3)(a),S_(k-3)(b), i.e., S_(k-3)(a)=s_(e)w₁ and S_(k-3)(b)=s_(o)w₂,respectively. Signals S_(k-3)(a), S_(k-3)(b) are also referred to hereinas OTD signals, and together signals S_(k-3)(a), S_(k-3)(b) collectivelycomprise an OTD pair.

[0026] For illustration purposes, the present invention will bedescribed herein with reference to STS and signals S_(k-2)( a), S_(k-2)(b). It should be understood that the present invention is alsoapplicable to OTD and signals S_(k-3)(a), S_(k-3)(b).

[0027] The output of processor 32 are signals s_(α-1), s_(α-2), wheresignal s_(α-1) comprises of signals S_(k-1) and S_(k-2)(a) and signals_(α-2) comprises of signals S_(k-2)( b), i.e.,s_(α-1)=ΣS_(k-1)+ΣS_(k-2)(a) and s_(α-2)=ΣS_(k-2)(b) . That is, signalsintended for STS compatible mobile-stations are included in both outputsignals s_(α-1), s_(α-2) and signals intended for non-STS compatiblemobile-stations are included in only signal s_(α-1). Alternately, signals_(α-1) comprises of signals S_(k-1) and S_(k-2)( b) and signal s_(α-2)comprises of signals S_(k-2)(a).

[0028] Signal s_(α-1) is split by splitter 34 into signals s_(α-1)(a),s_(α-1)(b) and processed along paths A1 and B1, respectively, bymultipliers 36, 38, 40, adders 42, 43 and amplifiers 44, 46 inaccordance with PSTD techniques. Signal s_(α-2) is split by splitter 35into signals s_(α-2)(a), s_(α-2)(b) and processed along paths A2 and B2,respectively, by multipliers 38, 40, 41, adders 42, 43 and amplifiers44, 46 in accordance with PSTD techniques. Note that signals s_(α-1)(a),s_(α-2)(a) are identical to respective signal s_(α-1)(b), s_(α-2)(b) interms of data, and that signals s_(α-1), s_(α-2) may be evenly orunevenly split in terms of power.

[0029] Signals s_(α-1)(b), s_(α-2)(b) are provided as inputs intomultipliers 36, 41 where signals s_(α-1)(b), s_(α-2)(b) are frequencyphase swept with phase sweep frequency signals (JIMMY: I can't edit theequations, but change all of the “−” signs in the exponents to “+” signsin ALL e^(j) terms. Please change this in all of the figures as well.e^(j) ^(Θ) ^(s) ^((t)) , e^(j) ^(Θ) ^(s2) ^((t)) to produce signalsS₃₆=s_(α-1)(b)e^(j) ^(Θ) ^(s) ^((t)) , S₄₁=s_(α-2)(b)e^(j) ^(Θ) ^(s2)^((t)) , respectively, wherein Θ_(s)=2πf_(s)t, e^(j) ^(Θ) ^(s) ^((t))=cos(2πf_(s)t)+jsin(2πf_(s)t), Θ_(s2)=−2πf_(s)t+π, e^(j) ^(Θ) ^(s2)^((t)) =−cos(2πf_(s)t)+jsin(2πf_(s)t), f_(s) represents a fixed orvarying phase sweep frequency and t represents time.

[0030] Note that phase sweep frequency signals e^(j) ^(Θ) ^(s) ^((t)) ,e^(j) ^(Θ) ^(s2) ^((t)) utilize a same phase sweep frequency with one ofthe signals, i.e., e^(j) ^(Θ) ^(s2) ^((t)) , rotating in the oppositedirection plus an offset of π relative to the other signal, i.e., e^(j)^(Θ) ^(s) ^((t)) . If the phase sweep frequency signals e^(j) ^(Θ) ^(s)^((t)) , e^(j) ^(Θ) ^(s2) ^((t)) were identical, i.e., Θ_(s)=Θ_(s2),self induced interference would be generated by base station 30 thatwould degrade STS/OTD performance. By configuring the phase sweepsignals e^(j) ^(Θ) ^(s) ^((t)) , e^(j) ^(Θ) ^(s2) ^((t)) to have thisrelationship, the self induced interference is canceled and STS/OTDperformance is optimized.

[0031] Signal S₄, is added to signal s_(α-1)(a) by adder 43 to producesignal S₄₃=S₄₁+s_(α-1)(a)=s_(α-2)(b)e^(j) ^(Θ) ^(s) ^((t)) +s_(α-1)(a).Signal S₄₃ and carrier signal e^(j2πf) ^(_(s)) ^(t) are provided asinputs into multiplier 40 to produce signal S₄₀, whereS₄₀=(S_(α-2)(b)e^(j) ^(Θ) ^(s2) ^((t)) +s_(α-1)(a)) e^(j2πf) ^(_(c))^(t), e^(j2πf) ^(_(c)) ^(t)=cos(2πf_(c) ^(t))+jsin(2πf_(c)t, and f_(c)represents a carrier frequency.

[0032] Signal S₃₆ is added to signal s_(α-2)(a) by adder 42 to producesignal S₄₂=s_(α-1)(b)e^(j) ^(Θ) ^(s) ^((t)) +s_(α-2)(a). Signal S₄₂ andcarrier signal e^(j2πf) _(c)t are provided as inputs into multiplier 38to produce signal S₃₈, where S₃₈=(s_(α-1)(b)e^(j) ^(Θ) ^(s) ^((t))s_(α-2)(a))e^(j2πf) ^(_(c)) t.

[0033] Signals S₄₀, S₃₈ are amplified by amplifiers 44, 46 to producesignals S₄₄ and S₄₆ for transmission over antennas 48, 50, where signalS₄₄=A₄₄((s_(α-2)(b)e^(j) ^(Θ) ^(s2) ^((t)) +s_(α-1)(a))e^(j2πf) ^(_(t))), S₄₆=A₄₆(s_(α-1)(b)e^(j) ^(Θ) ^(s) ^((t)) +s_(α-2)(a))e^(j2πf) ^(_(c))^(t), A₄₄ represents the amount of gain associated with amplifier 44 andA₄₆ represents the amount of gain associated with amplifier 46.

[0034] In one embodiment, the amounts of gain A₄₄, A₄₆ are substantiallyequal. In this embodiment, signals s_(α-1), s_(α-2) are split bysplitters 34, 35 such that the power levels of signals s_(α-1)(a),s_(α-2)(a) are substantially equal to the power levels of signals_(α-1)(b), s_(α-2)(b). Advantageously, equal gain amplifiers can beused, which lowers the cost of base station 30 compared to base stationcost when unequal amplifiers are used.

[0035] In another embodiment, the amounts of gain A₄₄, A₄₆ are differentand related to how splitters 34, 35 split signals s_(α-1), s_(α-2).Specifically, the amounts of gain A₄₄, A₄₆ applied to signals S₄₀, S₃₈should be amounts that would cause the power levels of signals S₄₄ andS₄₆ to be approximately or substantially equal. For purposes of thisapplication, power levels are “approximately equal” when the powerlevels are within 10% of each other.

[0036]FIG. 5 depicts a base station 70 employing symmetric sweep phasesweep transmit diversity in accordance with one embodiment of thepresent invention. In this embodiment, a form of PSTD referred to hereinas split shift PSTD in also utilized. Spilt shift PSTD involves shiftingboth signals split from a single signal using phase sweep frequencysignals that sweeps both signals in opposite direction. As shown in FIG.5, signals s_(α-1)(a), s_(α-2)(a) are phase swept by multipliers 37, 39using phase sweep frequency signals e−^(j) ^(Θ) ^(s) ^((t)) , e−^(j)^(Θ) ^(s2) ^((t)) , respectively. Although this embodiment depicts phasesweep frequency signals e^(j−) ^(Θ) ^(s) ^((t)) , e^(j) ^(Θ) ^(s) ^((t))equal and opposite to phase sweep frequency signals e^(j) ^(Θ) ^(s)^((t)) , e^(j) ^(Θ) ^(s2) ^((t)) , it should be understood that thephase sweep frequency signals used to phase sweep signals s_(α-1)(a),s_(α-2)(a) need not be equal in magnitude. In another embodiment,signals s., (a), s,2(a) are phase swept using phase sweep frequencysignals that result in phase swept signals s_(α-1)(a), s_(α-2)(a) with adesired or other phase difference to phase swept signals s_(α-1)(b),s_(α-2)(b). Note that that the phase sweep frequency signal used tophase sweep signals s_(α-1)(a), s_(α-2)(a), s_(α-1)(b), s_(α-2)(b) maybe phase shifting at an identical or different rate from each other, maybe phase shifting at fixed and/or varying rates, or may be phaseshifting in the same or opposite direction.

[0037] Although the present invention has been described in considerabledetail with reference to certain embodiments, other versions arepossible. For example, phase sweeping could be performed on paths A1and/or A2 instead of paths B1 and/or B2. In another example, the phasesweep frequency signals are interchanged. FIG. 4 depicts anotherembodiment of the present invention in which phase sweeping is performedalong paths Al and A2 instead of paths BI and B2 and phase sweepfrequency signals e^(j) ^(Θ) ^(s) ^((t)) , e^(j) ^(Θ) ^(s2) ^((t)) areprovided as inputs into multipliers 41, 36, respectively. Therefore, thespirit and scope of the present invention should not be limited to thedescription of the embodiments contained herein.

We claim:
 1. A method of signal transmission comprising the steps of:splitting a signal s₁ into signals s₁(a) and s₁(b), wherein signal s₁comprises a first STS/OTD signal belonging to an STS/OTD pair; splittinga signal s₂ into signals s₂(a) and s₂(b), wherein signal s₂ comprises asecond STS/OTD signal belonging to the STS/OTD pair; phase sweeping thesignal s₁(b) using a first phase sweep frequency signal to produce aphase swept signal s₁(b); phase sweeping the signal s₂(b) using a secondphase sweep frequency signal to produce a phase swept signal s₂(b), thefirst and second phase sweep frequency signals being configured tocancel out any self induced interference caused by phase sweeping thesignals s₁(b) and s₂(b); adding the phase swept signal s₂(b) to thesignal s₁(a) to produce a summed signal s₃; and adding the phase sweptsignal s₁(b) to the signal s₂(a) to produce a summed signal S4.
 2. Themethod of claim 1, wherein the first and second phase sweep frequencysignals utilize a same phase sweep frequency with the second phase sweepfrequency signal rotating in the opposite direction plus an offset of πrelative to the first phase sweep frequency signal.
 3. The method ofclaim 1, wherein the first and second phase sweep frequency signalsutilize a same phase sweep frequency with the first phase sweepfrequency signal rotating in the opposite direction plus an offset of πrelative to the second phase sweep frequency signal.
 4. The method ofclaim 1 comprising the additional steps of: amplifying the summed signals₃ to produce an amplified summed signal s₃; and amplifying the summedsignal s₄ to produce an amplified summed signal s₄.
 5. The method ofclaim 1 comprising the additional steps of: transmitting the summedsignal s₃ over a first antenna belonging to a pair of diversityantennas; and transmitting the summed signal s₄ over a second antennabelonging to the pair of diversity antennas.
 6. The method of claim 1comprising the additional steps of: prior to the step of adding thephase swept signal s₂(b) to the signal s₁(a), phase sweeping the signals₁ (a) using a third phase sweep frequency signal to produce a phaseswept signal s₁(a) with a different phase from the phase swept signals₂(b); and prior to the step of adding the phase swept signal s₁(b) tothe signal s₂(a), phase sweeping the signal s₂(a) using a fourth phasesweep frequency signal to produce a phase swept signal s₂(a) with adifferent phase from the phase swept signal s₁(b).
 7. A method of signaltransmission comprising the steps of: splitting a signal s₁ into signalss₁(a) and s₁(b), wherein signal s₁ comprises a first STS/OTD signalbelonging to an STS/OTD pair; splitting a signal s₂ into signals s₂(a)and s₂(b), wherein signal s₂ comprises a second STS/OTD signal belongingto the STS/OTD pair; phase sweeping the signal s₁(a) using a first phasesweep frequency signal to produce a phase swept signal s₁(a); phasesweeping the signal s₂(a) using a second phase sweep frequency signal toproduce a phase swept signal s₂(a), the first and second phase sweepfrequency signals being configured to cancel out any self inducedinterference caused by phase sweeping the signals s₁(a) and s₂(a);adding the phase swept signal s₂(a) to the signal s₁(b) to produce asummed signal s₃; and adding the phase swept signal s₁(a) to the signals₂(b) to produce a summed signal s₄.
 8. The method of claim 7, whereinthe first and second phase sweep frequency signals utilize a same phasesweep frequency with the second phase sweep frequency signal rotating inthe opposite direction plus an offset of π relative to the first phasesweep frequency signal.
 9. The method of claim 7, wherein the first andsecond phase sweep frequency signals utilize a same phase sweepfrequency with the first phase sweep frequency signal rotating in theopposite direction plus an offset of π relative to the second phasesweep frequency signal.
 10. The method of claim 7 comprising theadditional steps of: amplifying the summed signal s₃ to produce anamplified summed signal s₃; and amplifying the summed signal s₄ toproduce an amplified summed signal s₄.
 11. The method of claim 7comprising the additional steps of: transmitting the summed signal s₃over a first antenna belonging to a pair of diversity antennas; andtransmitting the summed signal s₄ over a second antenna belonging to thepair of diversity antennas.
 12. The method of claim 7 comprising theadditional steps of: prior to the step of adding the phase swept signals₂(a) to the signal s₁(b), phase sweeping the signal s₁(b) using a thirdphase sweep frequency signal to produce a phase swept signal s₁(b) witha different phase from the phase swept signal s₂(a); and prior to thestep of adding the phase swept signal s₁(a) to the signal s₂(b), phasesweeping the signal s₂(b) using a fourth phase sweep frequency signal toproduce a phase swept signal s₂(b) with a different phase from the phaseswept signal s₁(a).
 13. A base station comprising: a first splitter forsplitting a signal s₁ into signals s₁(a) and s₁(b), wherein signal s₁comprises a first STS/OTD signal belonging to an STS/OTD pair; a secondsplitter for splitting a signal s₂ into signals s₂(a) and s₂(b), whereinsignal s₂ comprises a second STS/OTD signal belonging to the STS/OTDpair, a first multiplier for phase sweeping the signal s₁(b) using afirst phase sweep frequency signal to produce a phase swept signals₁(b); a second multiplier for phase sweeping the signal s₂(b) using asecond phase sweep frequency signal to produce a phase swept signals₂(b), the first and second phase sweep frequency signals beingconfigured to cancel out any self induced interference caused by phasesweeping the signals s₁(b) and s₂(b); a first adder for adding the phaseswept signal s₂(b) to the signal s₁(a) to produce a summed signal s₃;and a second adder for adding the phase swept signal s₁(b) to the signals₂(a) to produce a summed signal s₄.
 14. The base station of claim 13,wherein the first and second phase sweep frequency signals utilize asame phase sweep frequency with the second phase sweep frequency signalrotating in the opposite direction plus an offset of π relative to thefirst phase sweep frequency signal.
 15. The base station of claim 13,wherein the first and second phase sweep frequency signals utilize asame phase sweep frequency with the first phase sweep frequency signalrotating in the opposite direction plus an offset of π relative to thesecond phase sweep frequency signal.
 16. The base station of claim 13further comprising: a first amplifier for amplifying the summed signals₃ to produce an amplified summed signal s₃; and a second amplifier foramplifying the summed signal s₄ to produce an amplified summed signals₄.
 17. The base station of claim 13 further comprising: a pair ofdiversity antennas having a first and a second antenna; a firsttransmitter for transmitting the summed signal s₃ over the firstantenna; and a second transmitter for transmitting the summed signal s₄over the second antenna.
 18. The base station of claim 13 furthercomprising: a third multiplier for phase sweeping the signal s₁(a) usinga third phase sweep frequency signal to produce a phase swept signals₁(a) with a different phase from the phase swept signal s₂(b) prior toadding the phase swept signal s₂(b) to the signal s₁(a); and a fourthmultiplier for phase sweeping the signal s₂(a) using a fourth phasesweep frequency signal to produce a phase swept signal s₂(a) with adifferent phase from the phase swept signal s₁(b) prior to adding thephase swept signal s₁(b) to the signal s₂(a).
 19. A base stationcomprising: a first splitter for splitting a signal s₁ into signalss₁(a) and s₁(b), wherein signal s₁ comprises a first STS/OTD signalbelonging to an STS/OTD pair; a second splitter for splitting a signals₂ into signals s₂(a) and s₂(b), wherein signal s₂ comprises a secondSTS/OTD signal belonging to the STS/OTD pair; a first multiplier forphase sweeping the signal s₁(a) using a first phase sweep frequencysignal to produce a phase swept signal s₁(a); a second multiplier forphase sweeping the signal s₂(a) using a second phase sweep frequencysignal to produce a phase swept signal s₂(a), the first and second phasesweep frequency signals being configured to cancel out any self inducedinterference caused by phase sweeping the signals s₁(a) and s₂(a); afirst adder for adding the phase swept signal s₂(a) to the signal s₁(b)to produce a summed signal s₃; and a second adder for adding the phaseswept signal s₁(a) to the signal s₂(b) to produce a summed signal s₄.20. The base station of claim 19, wherein the first and second phasesweep frequency signals utilize a same phase sweep frequency with thesecond phase sweep frequency signal rotating in the opposite directionplus an offset of π relative to the first phase sweep frequency signal.21. The base station of claim 19, wherein the first and second phasesweep frequency signals utilize a same phase sweep frequency with thefirst phase sweep frequency signal rotating in the opposite directionplus an offset of π relative to the second phase sweep frequency signal.22. The base station of claim 19 further comprising: a first amplifierfor amplifying the summed signal s₃ to produce an amplified summedsignal s₃; and a second amplifier for amplifying the summed signal s₄ toproduce an amplified summed signal s₄.
 23. The base station of claim 19further comprising: a pair of diversity antennas having a first and asecond antenna; a first transmitter for transmitting the summed signals₃ over the first antenna; and a second transmitter for transmitting thesummed signal s₄ over the second antenna.
 24. The base station of claim19 further comprising: a third multiplier for phase sweeping the signals₁(b) using a third phase sweep frequency signal to produce a phaseswept signal s₁(b) with a different phase from the phase swept signals₂(a) prior to adding the phase swept signal s₂(a) to the signal s₁(b);and a fourth multiplier for phase sweeping the signal s₂(b) using afourth phase sweep frequency signal to produce a phase swept signals₂(b) with a different phase from the phase swept signal s₁(a) prior toadding the phase swept signal s₁(a) to the signal s₂(b).